1. Field of the Invention
This invention relates to off-the-road heavy duty pneumatic radial tires. More particularly it relates to an off-the-road heavy duty pneumatic radial tire which can attain the improvement of stable running performances by lessening the lateral rolling of the vehicle and the longitudinal pitching on unevenness of road surface when the vehicle is run on severely rough roads such as mines, forests, wild plains, digged ground or the like under full carrying load of heavy bulks such as ores, earth and sand or the like, and can simultaneously improve the carcass durability at the bead portion.
2. Related Art Statement
In extra-heavy load vehicles such as a large size dump truck or the like, when the vehicle is run on rough road, the tire treads in concave portions of the rough road at a side in the running direction or rides on convex portions and further is subjected to a large lateral force in the turning or the like, whereby a considerably large rolling feice is caused in the vehicle to undesirably drop a large amount of the carried earth and sand from the vehicle on a side of the running road.
On the other hand, the tire repeatedly generates a large deformation even in the longitudinal direction through the unevenness of the road, whereby the pitching is caused in the vehicle. In case of a rear dump truck, the carried load undesirably drops backward.
When the material dropped exists on the running road, there is a fear of causing an unexpected accident upon the riding of the following vehicle on the dropped material. In order to avoid the occurrence of such accidents, the work for removing the dropped matter is frequently conducted, which increases the maintenance cost.
As the tire for this type of the vehicle, lateral rigidity enough to resist against a large lateral force generated due to the unavoidable unevenness of road or turning is required for the resistance to lateral rolling, while a followability causing an adequate buffering action is required against the deformation in longitudinal direction. When the rigidity of the sidewall portion is sufficiently increased for ensuring the resistance to lateral rolling, the resistance to deformation in the longitudinal direction of the tire tends to excessively increase, whereby pitching becomes violent, so that the resistance to rolling conflicts with followability.
Therefore, in order to simultaneously establish resistance to lateral rolling and the proper followability in the running on rough road, it is important to ensure the stable running performances or the stability performances. At the same time it is required to have an excellent carcass durability performance in the bead portion. That is, the tire is demanded to simultaneously possess these performances.
In order to improve the above stability performances, it has hitherto been attempted to arrange a portion having a thicker rubber gauge in the main carcass portion of the carcass ply, particularly at an inside of the sidewall region. In this case, however, the followability of tire deformation on road surface is poor when the tire is run a on severely rough road. Furthermore, since the rubber gauge is preferentially increased in the sidewall region, the load bearing of the bead portion in the tire increases and the tendency of heat build up becomes strong to degrade the carcass durability in the bead portion. Moreover, the load bearing at both side end portions of the belt increases to lower the resistance to separation failure at the side ends of the belt. Further it is undesirably obliged to increase the tire weight.
On the other hand, Japanese Patent Application Publication No. 56-28721 discloses a heavy duty pneumatic radial tire, wherein a turnup end of a turnup portion of a carcass ply is made higher a position of maximum width of the carcass ply A flexible rubber layer having a JIS hardness of about 46.degree. (corresponding to Shore A hardness of 48.degree.) is interposed between the main carcass portion and the turnup portion in the carcass ply to separate the main carcass port-ion and the turnup portion from each other. By this technique the enhancement of the rigidity of the sidewall portion and the improvement of the durability in the bead portion can be realized.
In this tire, a flexible rubber having a triangular shape in section or a so-called cushion rubber having, for example, a JIS hardness of 46.degree..about.48.degree. is arranged between each side end portion of the belt and the carcass ply for mitigating the shearing stress, which is not clearly disclosed in the above publication, and further a flexible rubber having, for example, a JIS hardness of about 42.degree. is generally used as a sidewall rubber over a region ranging from the bead portion to the buttress portion for ensuring the resistance to flex fatigue. Such a structure is suitable in view of the followability on road surface in the running on rough road. However, large deformation is particularly caused at the buttress portion near the ground contact region through lateral force generated in the running of the tire as compared with the case of applying no lateral force to stagger the tire in the axial direction, so that the degradation of resistance to lateral rolling can not still be avoided.
In general, the heavy duty pneumatic radial tire is repeatedly subjected to a large deformation in the curved region of the bead portion along the rim under an influence of a load acting to the tire, and the cord break-up of the carcass ply (hereinafter referred to as CBU) is apt to be caused in the inside of the bead portion through compression strain. However, a special countermeasure for such a problem is not taken in the tire.